Photomask flatness is an important concern in the fabrication and application of photomasks in lithography for semiconductor manufacturing. With the advancement of technology, feature sizes are decreasing, thus requiring a larger numerical aperture (NA) that results in smaller depth of focus. As a result, photomask flatness is no longer a negligible component because a slight curvature will not only reduce the depth of focus, but also create a lateral image shift.
Recently, studies have shown that photomask pellicles play an important role in impacting the final photomask flatness. These studies demonstrate that the attachment of a mask pellicle can distort the final mask due to the interaction between the mask shape and the pellicle frame shape. The attachment of the pellicle frame has been shown to cause an additional distortion averaging 300 nm total indicated readout (TIR) of point to point difference from the patterned mask. In addition, pellicle attachment often causes sharp, localized shape changes within the print area. The flatness deviation after pellicle attachment depends on the pellicle frame flatness and the shape matching to the photomask flatness. Therefore, reducing these pellicle effects can insure flatter, more uniform masks resulting in improvement in printed wafer quality.